Alright, so basically it's Super Metroid, but holy hell it's really well done. I like where this is going.
The art style and atmosphere are great. I really like the little touches, like how water and goo moves and the swaying of plants when you walk by.
I thought the oxygen limit would be frustrating, but there are only select rooms that don't have air, and the mechanic adds some urgency to the gameplay. You get enough time to explore, but there's a sense that you need to keep moving to stay safe. I'm guessing this might get expanded upon in the full release.It also adds to the feeling that you're not some superhuman, but a person stuck in a messed up situation.
The only issue I think is the amount of health the boss has. It takes a really long time to kill and the flying one hit creatures don't give a lot of missiles. Then again, the missiles take off quite a big chunk of health off the boss, so it's probably for balance.